Genetic exploration of a nuclear receptor transcriptional regulatory complex

Metazoan transcriptional regulatory factors (TFs) bind to genomic response elements and assemble with co-regulators into transcriptional regulatory complexes (TRCs) whose composition, structure and activities are gene-, cell- and physiological-context specific. Each TRC is a "regulatory logic module," integrating incoming signaling information, which defines context and thereby recruits a distinct combination of co-regulators that together specify outgoing regulatory activity. Analyzing TRCs unique to every context is daunting, yet justified by their properties as self-contained regulatory modules. As proof-of-concept, we performed a forward genetic screen in C. elegans carrying a synthetic simple response element for nuclear receptor NHR-25 upstream of a fluorescent reporter gene. We isolated independent mutations in uba-2, a component of the sumoylation signaling machinery, and in lir-2, which we demonstrated to be a novel co-regulator, interacting with NHR-25 through LxxLL motifs and modulating target gene expression. Our studies establish that an unbiased genetic screen readily identifies both afferent and efferent components that specify TRC function, and suggest that screening natural response elements of interest could illuminate molecular mechanisms of both context-specificity and transcriptional regulation.